Thermal switch



July 27, 1937. E. J. scHAEFER THERMAL SWITCH Filed Sept. 18, 1935 Inventor: Edward J ScznaLeeeI mj 6.3M bg Hl Attorneg.

Patented July 27, 1937 UNITED STATES 2,088,443 THERMAL SWITCH Edward J. Schaefer, Fort Wayne, Ind., assignor to General Electric Company, a corporation of New York Application September 9 Claims.

My invention relates to thermal switches, more particularly to overload l responsive thermal switches, and has for its object the provision. of a simple, reliable and low cost device of this character.

My invention is especially useful in the protection of small electric motors of fractional horsepower size, it being contemplated that the device, lfor purposes vof convenience, will be mounted on the frame of the motor itself, although it may of course be mounted in any convenient location.

AIn carrying out my invention in one form I provide a bow-shaped snapping spring for controlling the switch contacts together with a bimetallic thermostat shaped in the form `of a helix for operating the snapping spring from one position to another. This thermostat is heated in response to the current in the switch or motor circuit by means of a helically shaped resistor surrounding the thermostat. The thermostat may have one end secured to the base and its other end rotatable in response to temperature changes to snap the spring from the closed to the open position and vice versa.

Orr the other hand, one end of the helical thermostat may beprovided with limited freedom of rotation so that the thermostat itself may be rotated bodily by a suitable handle to open and close the circuit under predetermined normal temperature conditions. In such case,

the rotation of the thermostat with respect to the base is limited by a stop,A however, so that upon a predetermined change in temperature the thermostat operates the snapping member to control the switch contacts.

For a more complete understanding of my invention, reference should be had to the accompanying drawing, Fig: 1 of which is a plan view of a thermal switch embodying my invention;

Fig. 2 is a sectional view along the `line 2-2 of Fig. 1 looking in the direction of the arrows;

Fig. 3 is a sectional View along the -line 3-3 of Fig. 1 looking in the direction of the arrows; Fig. 4 is a sectional view along the line 4-4 of Fig; 1 looking in the direction of the arrows; Fig. 5 is a fragmentary view showing the details of the snapping spring and its connections with the thermostat and switch arm, while Fig. 6 is a fragmentary view` showing a modified form of my invention provided with a handle or knob for manual operation of the switch contacts.

Referring to the drawing, in one embodiment of my invention, I have shown a. flexible metal contact arm I0 preferably made of vphosphor 18, 1935, Serial No. 41,059

bronze material, having one end clamped by a screw I I to the insulating support or base I2 and carrying near its other end a switch contact I3 co-operating with a similar Switch contact I4 carried by a conducting bar I5 which is secured 5 to the base by a screw I6. The free end of the switch arm I0 terminates in a pair of parallel spaced projections I1 which embrace a snapping spring I8 at a point near its middle whereby snapping movement of the spring operates the l0 movable contact I3.

As shown perhaps most clearly in Fig. 5, the snapping spring I8 is mounted between the arms of a U-shaped metal support I9 which is suitably secured in a recess provided for it in the base l5 I2. The spring is a steel strip which is normally straight but is compressed between the arms of the support I9 into a bow shape. The spring thus has two stable positions -in one of which, as shown in Fig. 5, the movable contact I3 is in 20 the open circuit, while in the other stable position of opposite curvature, as indicated in Fig. 3, the switch contacts are in engagement. Fur.- thermore, when the spring is moved from one stable position to the other, it operates after 25 initial compression with a snap action to the opposite position, thus operating the switch contacts with a snap movement. K

For operating the spring I8, I have provided a bimetallic thermostat 20 which is wound into 30 the shape of a helix, one end 2| being secured to the base, while the other end is free to rotate as the helix uncoils or coils up in response to changes in temperature. This free end has secured rigidly to it an operating arm 22 which is' 35 offset or extends laterally with respect to theaxis of the thermostat helix. At its end, the arm 22 is bent laterally so as to extend at right angles to the spring I8, this portion being slotted or in other words consisting of two parallel projections 23 which embrace the spring I8 at a point near one end of the spring. It will thus be. observed that rotation of the free end of the thermostat through a small angle actuates the arm 22 and through it snaps the spring from one position vto the other. To facilitate rotation of this free end an internal bearing 24- is provided for it, this bearing consisting of a sleeve which is mounted rigidly on a projection 25 extending laterally from one arm of the U- shaped support I8. The bearing 24 co-operates S with a suitable bearing sleeve 26 secured to the thermostat.

The thermostat is heated by means of a resistor 21 formed from a strip of suitable resistv mounted in the base below the resistor and extending lengthwise and laterally of the base to..

terminate in a connection screw 3i. A sheet 32 of insulating material, such as mica, is placed over the conducting strap lll. The connections' for the ends of the resistor serve also to support the resistor.

'Ihe circuit through the switch leads from the binding screw I5, for example, through the contacts when the switch is' closed, the contact arm to the end 28 of the resistor, and thence through the resistor, the conducting strapl, and out through thefbinding. screw Il. It is essential that the resistor helix be large enough to assure a slight spacing between it and the inner thermostat helix whereby the two helixes` are electrically insulated from each other.

Since the two hellxes when formed have sumclent rigidity to be amply self-supporting, a relatively small spacing is suillcient. Furthermore,

the internal bearing 24 for the free end of the thermostat serves to accurately position the thermostat.

The end 2l of the thermostat which is secured to the base may be adjustable, i. e., rotated slightly to eiect a temperature adjustment of the switch. As shown in Fig, 4, for that purpose this end of the thermostat is held in a recess provided for it in the base by means of a clamping strap 3l having a protuberance or pin Il which ts in an aperture provided for it in the thermostat. v4The strap Il is in turn secured by the clamping screw Il. In order to change the temperature adjustment, the screw Il is loosened and the thermostat turned by sliding the clamping strap I3, an elongated hole 33 being provided in the strap for the clampswitch. As shownfthe knob 3l is secured to an operating shaft It which is rotatably mounted in the wall of the base. and has its inner end extending for a4 short distance into the thermostat helix and secured rigidly to it,'for example, by brazing or by clamping the two parts together. lThe operating shaft 3G carries a pin 3l which extends upwardly into a slot ll provided in a clamping strap Il which corresponds with the strap of Fig. 4. In this case, however, the strap 39 does not clamp the thermostat rigidly to the base but is sumciently loose to allow the thermostat to be' turned freely by the knob 3l within the limits prescribed by the slot Il. f

Thus the pin Il 'ensginilthe end walls of the slot Il limits the bodily rotational movement lswitch to the open position.

of the thermostat to a small arc. This move- .ment is. just sufilcient to stress the spring I8 and snap it to its opposite position, the stop preventing excessive strains in the thermostat and distortion `thereof.

Ihe strap 39 also serves the purpose of securing the thermostat against ,rotation to enable it to operate the `snapping spring in response to changes in temperature. The pin 31 under normal temperature conditions, i`. e., with the thermostat at substantially its ambient temperature, occupies a position near the middle of the slot It when the switch is either open or closed as indicated in Fig. 6. Assuming that the switch is closed, upon the occurrence of 'predetermined overload current for which the device ls adjusted, the thermostat uncoils, the pin Il moving toward the right hand end wall .of the slot until finally it engages theend wall and thenfurther distortion oi the thermostat causes the opposite end to rotate and snap the 'I'he switch must be reclosed by means of the knob 35.

A turther advantage of this arrangement arises from the fact that after opening on overload the switch cannot be reclosed by turning the handle -until the thermostat has cooled to nearly the ambient temperature. In other words, any movement of the handle to reclose, which must be in a direction to move the pin toward the right hand end of the slot, based on the assumptions previously made, has no effect on the snapping spring until the thermostat has cooled and substantially recoiled itself. A suitable cover is provided for the base, then two parts preferably being molded from a suitable electrically insulating material such as a phenolic condensation product.

While I have shown a particular embodiment of my invention, it will be understood of course that I do not wish to be limited thereto since many modifications may be made, and therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent o! the United States is:

.1. A thermal switch comprising a base, an

being provided with a recess adjacent the movable ends of said thermostatand said. contact arm and extending transversely thereto, a normally straight spring', abutments at the ends loi' said transverse recess between which said spring is supported and compressed into a bow shape,

secured to said base arranged to be engaged 75 by said movable contact in one of the positions of said contact arm.

2. A thermal switch comprising a base, a helically shaped bimetallic thermostat having one end secured to said base, a flexible contact arm having one end secured to said base adjacent said secured end of said thermostat and extending in parallel relation with the thermostat, said thermostat and said contact arm being of substantially the same length, said base being provided with a recess adjacent the movable ends of said thermostat and said contact arm, and extending transversely thereto, a normally straight spring, abutments at the ends of said transversel recess between which said spring is supported and compressed into a bow shape, said spring extending across the movable ends of said thermostat and said contact arm, an operating arm on the movable end of said thermostat, a pair of projections on the end of said arm extending at right angles to said spring and embracing said spring at a point between its ends whereby rotation of the movable end of said thermostat in response to changes in temperature snaps said spring from one position to another, a pair of spaced projections on the movable end of said contact arm embracing said spring between its ends whereby snapping movement of said spring operates said arm from one position to another, a movable contact secured to said contact arm near its movable end, a contact secured to said base arranged to be engaged by said movable contact in one of the positions of said contactl arm, a helical heating resistor surrounding said thermostat in spaced relation therewith, a connectiony between one end of said heater and the secured end of said contact arm, a pair of terminal members secured to said base and electrical connections respectively between said terminal members and said stationary contact and the other end of said heater.

3. A thermal switch comprising an elongated base made of electrically insulating material provided with a longitudinally extendingrecess at one side and a transversely extending recess at one end, a helically shaped bimetallic thermostat in said longitudinally extending recess, a flexible contact arm, a screw clamping one end of said contact arm to said base, said con! tact arm extending in parallel relation with said thermostat, said thermctstat and said contact arm being of substantially the same length, al clamping strap extending across the xed ends of said thermostat and said contact arm, said strap having one end secured to said base by said screw and its other end clampinghone end of said thermostat` to said base, an operating connection between said strap and said thermostat for securing said thermostat against rotation, said strap being slidable on said base by loosening said screw to adjust the angular position of said thermostat, a normally straight spring strip supported between thevends of said transverse recess and compressed thereby into a bow shape, an operating arm on the movable end of said thermostat, projections on the end of said arm extending at right angles to said spring strip and embracing said strip whereby rotation of the movable end of said thermostat upon changes in temperature snaps said spring* from one position to another, spaced projections on the movable end of said switch arm embracing said spring strip whereby the switch arm is moved from one position to another by the snapping of said spring strip, a contact carried by the switch arm near its movable end, and a contact secured to the base in position to be engaged by said movable contact in one position of said switch arm.

4. A thermal switch comprising an elastic snapping element, switch contacts operated by said snapping element, a support, a temperature responsive actuating element mounted for bodily movement on said support, a connection between said temperature responsive element and said snapping element, a manually operable means for moving said temperature responsive element on said. support to actuate said elastic element under normal temperature conditions, and a stop` on said support co-operating with said temperature responsive element to limit said bodily movement of said temperature responsive element to the range of movement required` to actuate said elastic 'element and thereby secure said element for operation of said elastic element by said temperature responsive element in response to a predetermined change in temperature. h

5. A thermal switch comprising a support, a snapping spring. mounted on said support, a bimetallic thermostat connected to operate said spring, means mounting said thermostat on said support for movement bodily with relation to said support to operate said spring, and a stop on said support cooperating with said thermostat for limiting the movement of said thermostat on said' l an operating connection between said spring and said switch arm, a helically shaped bimetallic thermostat, means mounting said thermostat for y rotation on said support, a connection between one end of said thermostat and said spring whereby rotation of said end .through a predetermined angle snaps said spring from one position to another, a handle connected to the opposite end of said thermostat for rotating said thermostat bodily to snap said spring at normal temperatures and thereby operate said switch arm, a stop on said support cooperating with said thermostat tov limit the angular movement of the said last-mentioned end of said thermostat to an amount not substantially greater than said `angle to thereby secure said thermostat for actuation of said spring in response to a predetermined change in temperature, said stop preventing subsequent operation of said spring by means on said support i'or limiting said bodily movement of said temperature responsive element A on said support for bodily movement with relation to said support to actuate said snapping element under normal temperature conditions, an

operating handle connected to said temperature responsive element for moving said element bodily to actuate said snapping element, and stop means on said base for limiting said bodily movement ot said temperature responsive element to the range of movement required to actuate said snapping element under normal temperature conditions and thereby secure said temperature responsive element for actuation oi.' said snapping element by distortion of said temperature responsive element in response to a predetermined change in temperature.'

9. A thermal and manually operated switch comprising a temperature responsive element, a

- control device operated thereby, a support, means mounting said element on said support for limited freedom of movement bodily over a predetermined range on said support, an operating handle connected to said element for moving said element bodily with relation to such support over said predetermined range. said range of movement oi' said element on said support being such that said control device can be operated at a predetermined temperature by bodily movement of said element by said handle and such that upon distortion of said thermostat in response to a change in temperature said thermostat issecured at one limit of its range of movement for operation oi' said control device by said thermostat at a diierent predetermined temperature.

EDWARD J. SCHAEFER. 

